1. Field of the Invention
This invention relates to orbital polishing and more particularly to an apparatus and method for polishing a workpiece utilizing an orbital polishing machine with a flowable elastic abrasive media.
2. Description of Related Art
Abrasive flow machining is a well known non-traditional machining process whereby a visco-elastic media permeated with an abrasive grit is extruded through or past a workpiece surface to abrade surfaces over which the media passes. The abrasive action and abrasive flow machining can be thought of as analogous to a filing, grinding, lapping or honing operation where the extruded visco-elastic abrasive media passes through or past the workpiece as a “plug.” The plug then becomes a self-forming file, grinding stone or lap as it is extruded under pressure through or past the workpiece, thereby abrasively working upon selected surfaces of the workpiece. The special properties of the visco-elastic media are such that the material will become most aggressive at its greatest restriction.
Recently, this technology has been utilized with orbital polishing to create a hybrid technology. Orbital polishing uses much of the same technology as abrasive flow machining, but utilizes a mechanical motion to polish three-dimensional forms not possible to be polished by the conventional abrasive flow machining technique. While traditional abrasive flow machining requires flow of abrasive media over or through the workpiece, such flow may or may not be used with the orbital polishing process, since motion is imparted to the abrasive media by the orbital polishing machine independent of any abrasive media flow. Details of an orbital polishing machine may be found in U.S. Pat. No. 4,891,916, assigned to Extrude Hone Corporation, the assignee of the present invention and the contents of this patent are herein incorporated by reference.
The flowable abrasive media may be a flowable, visco-elastic polymer with abrasive particles mixed therein. The polymer and abrasives selected for a given polishing operation are determined in part by the material to be polished and the polish finish desired. Typical abrasives include silicon carbide, aluminum oxide and boron carbide. The use of such media for metalworking operations is described in detail in U.S. Pat. No. 6,273,787, assigned to Extrude Hlone Corporation, the assignee of the present invention and the contents of this patent are hereby incorporated by reference.
One area of manufacturing in which orbital polishing has been used only to a limited amount is for the polishing of motor vehicle wheels. Many of these wheels have decorative, highly polished shapes which are very difficult to polish into a finished product. Manufacturers of after-market motor vehicle wheels are presently limited in their production volumes due to limitations required by polishing such wheels and the existence of few facilities capable of supporting the industry. The current production volumes of a given wheel range from hundreds of sets to thousands of sets per month. Typically, these wheels are polished by hand, although some other semi-automated process, such as a buffing process, may be used as well. Hand polishing generates high demands on the existing labor force and the semi-automatic process generates high demands on existing equipment. Current equipment and processes also generate high amounts of air-borne pollutants. The demand for such polished wheels currently exceeds the supply by up to 30% since such production increases have not been possible because of lack of equipment and a process to polish cast wheels in a more timely and cost effective manner.
Whenever the wheels are polished by hand, the finish of such a polishing effort is dependent on the skill of the individual person and, as a result, each wheel will have a variance from the next wheel for a given size. Another prior art polishing process involves a shot peening process where a media is forced into contact with a wheel and the wheel is set in motion in an effort to polish it. In other mechanical processes, buffing and/or brushing are utilized. The shot peening process does not produce the desired highly polished surface. With the buffing process, the complexity of some of the wheels is such that the buffing wheels do not extend into the intricate pattern of the face of the wheel. The buffing wheel will only reach limited areas of the wheel, thereby requiring the use of hand finishing to achieve the final luster. Nevertheless, it is impossible to create a consistently “leveled” surface finish on a wheel with any current process.
In general, prior polishing systems have failed to achieve the desired cycle times with the desired efficiency. Furthermore, the size and operational dynamics of a traditional abrasive flow machine for polishing wheels larger than 20 inches in diameter, such as wheels for over-the-road vehicles, makes the use of existing AFM techniques impractical, since such techniques are intended for much smaller workpieces.